But do the maths: 4 x sqns of 4 x troops of approx 16. Add HQ, trg, and that's under 400.
Not only is there no direct entry, but there is no "guaranteed" career in the regt on passing selection, for JNCOs or officers. A good 50% of JNCOs return to their parent regt. on completing 3 years. Even those who stay at that point do not get on the Regt Cadre for another few years.
Officers get a troop command, some come back for a Sqn command, three get a Regt command. (The two TA Regts have reg Lt Cols as CO).
From a career perspective I'd agree with others and suggest RM as being an excellent career choice for a young wannabe officer. Second option would be Royal Engineers, who are superb in my experience, and have Para and Marine elements.